This invention relates to solid state power control devices such as solid state electric relays wherein it is desired to obtain electric isolation and optical coupling between a high power controlled circuit and a low power control circuit. Typically, the invention is applicable to thick film hybrid devices comprising a ceramic substrate with a thick film controlled circuit adhered to it and with at least one light activated switch in the circuit which is to be activated by a light emitting element such as a light emitting diode (LED) that is to be electrically isolated but optically coupled with the switch element. The term "light" is used herein in a broad sense and is intended to encompass radiation within the visible wavelength band as well as outside of this band such as in the ultraviolet and infrared bands.
As is well known, solid state control devices such as relays comprise thick film conductors adhered to a surface of a ceramic substrate with passive and active circuit elements connected between the conductors. Typically, such devices are fabricated by printing thick film conductive bands on a selectively metallized substrate using a slurry of conductive particles which is then dried. Passive elements such as resistors are then formed by printing with a slurry of more resistive particles then fired at a temperature of 850.degree. C. to 1100.degree. C. A lead frame comprised of joined thin metal strips is then deposited on certain of the conductive bands. Prefabricated capacitors and active elements such as transistors, silicon controlled rectifiers, diodes and triacs on which solder or other fusible material has been previously applied are deposited at appropriate places relative to the printed circuits and lead frame. The interfaces of the components are metallized such as with solder. The whole assembly is then heated, usually to a maximum of 350.degree. C., to cause the fusible metallizing material to reflow and effect a bond between the parts which becomes rigid after the device is cooled. An illustrative thick flim device may be seen in U.S. Pat. No. 3,958,075 which is assigned to the assignee of this application.
Solid state relays often have at least one light receiver such as a light activated switch operating in a comparatively high voltage and high current load circuit. High power load or controlled circuits of this type are often activated with a light emitting device such as a light emitting diode (LED) which is in a low voltage control circuit and which must be electrically isolated from, but optically coupled with, the light activated element. The low voltage control circuit usually has other elements such as diodes and resistors in series with the LED. Prior practice has been to deposit the elements of the control circuit on the substrate such that the LED was positioned where it would be optically coupled with the light activated element in the load circuit. This method of construction required several of the reflowing or fusing operations which were mentioned above. When using this method, it is difficult to get the light emitting and light receiving circuit elements accurately aligned for obtaining optimum coupling and, hence, sensitivity.
Heretofore, in order to achieve optical coupling and electric isolation in a hybrid circuit, three choices were available. One was to use a discrete photon or light coupled pair package and to mount this package in whatever space was available on the substrate. Another choice was to use two independent substrates using planar coupling with a reflective dome. Another choice was to use an LED combination on a perpendicular structure with a glass type interface isolation wherein the elements were mounted in a single plane. All of these choices had the disadvantages of either requiring multiple reflow operations or requiring a substantial amount of space.